What does underweight mean?

If you are underweight, it means that your body weight is below the healthy weight range — the weight that research has shown you need to be for your body to work at its best.

Being outside this range, whether overweight or underweight, can affect aspects of your health such as your:

• immunity
• fertility
• mental wellbeing

You might have difficulty gaining weight when you want to.

How do I know if I am underweight?

A good measure of whether you are a healthy weight is the body mass index (BMI). This is calculated using your weight (in kilograms) and your height (in metres squared). For most adults, a healthy weight range is when your BMI is between 18.5kg/m2 and 24.9kg/m2.

If your BMI is under 18.5kg/m2 then you would be considered underweight.

However, BMI calculations do not apply in the same way for everyone.

These people should not rely on their BMI to calculate if they are underweight:

• athletes
• pregnant people
• older people
• people of some non-white ethnic backgrounds. For example, people of Asian background generally have a lower healthy weight BMI and people of Polynesian background generally have a higher healthy weight BMI.

If you think you might be in one of these groups, ask your doctor for advice about a healthy weight range.

Children's weight is usually measured using growth charts, rather than BMI. Growth charts check how a child is growing against what would be expected, based on other children of the same sex and age.

If you're worried that your child may be underweight, speak to their doctor or Maternal and Child Health nurse.

What are the symptoms of being underweight?

If you are underweight, you might find you get sick often or feel very tired. That may be because you are not getting enough of the nutrients (food groups) you need from your diet.

You may also find that:

• your hair gets thinner or falls out
• your skin gets very dry
• you have problems with your teeth

Children who are underweight may not grow as expected for their age.

Being underweight can lead to health problems including:

• osteoporosis
• infections, which you might find harder to fight off than other people
• anaemia
• irregular periods in females, or menstruation may stop altogether
• pregnancy problems, such as difficulty getting pregnant or a higher chance of preterm labour
• heart problems

Being underweight can be especially dangerous for older people since it increases your risk of breaking bones and becoming ill.

What causes being underweight?

There are many possible causes for being underweight. You may be born with a naturally small body frame, and your low BMI can be due to your genes. You may have a very high metabolism and find it hard to put on weight, even if you eat foods that have a lot of energy (kilojoules).

There are also some physical conditions that can cause weight loss, leading to underweight. These include:

• digestive problems, such as nausea, vomiting or diarrhoea
• an infection
• cancer, HIV or lung disease
• diabetes
• thyroid problems
• Crohn's disease

Some people find it hard to have a healthy, balanced diet because of the cost of healthy food or missing meals due to busy schedules.

It can be hard to eat properly if you are sick, or if your medicines make you lose your appetite or feel nauseous (that you might vomit). If you do a lot of physical activity you may burn up more energy than you can eat, leading to you being underweight.

Mental health problems can also cause you to be underweight. These include:

• anxiety and stress
• eating disorders such as bulimia and anorexia nervosa
• dementia

People who have been ill, in hospital or who are older have an increased chance of being underweight.

When should I see my doctor?

If your BMI is under the healthy weight range, you are losing weight without trying to or are having difficulty gaining weight, you should see your doctor.

If you are worried that you or someone you care for may have an eating disorder, talk to your doctor — it is important to seek treatment from a health professional.

Support is also available online for people with eating disorders.

How can I gain weight safely?

If you are underweight, it's important to eat a variety of foods that give you the nutrition you need. You should try to eat foods with enough:

• energy (kilojoules) to help you gain weight
• protein to repair your body and build your muscles
• vitamins and minerals to keep your body cells functioning well

The Australian dietary guidelines can help you learn about the amount and kinds of foods that you need to eat for health and wellbeing. By following the dietary patterns recommended in the guidelines, you will get enough of the nutrients essential for good health. Eating well can also help reduce your chance of long-term health problems.

You can put on weight by eating small meals frequently throughout the day. Try to snack on healthy, high-energy foods like cheese, nuts, milk-based smoothies and dried fruit.

More tips for gaining weight safely include:

• using full cream milk in coffee, tea or with meals
• using lots of healthy fats like olive oil or avocado in your meals
• sprinkling some grated cheese on cooked food
• adding milk powder to soups, stews and drinks like milkshakes
• adding protein powder to milkshakes
• replacing black tea or coffee with milky drinks
• doing some light exercise to increase your appetite

If you have difficulty finding the time to prepare meals each day, it can be useful to have meals delivered or to prepare a few meals in advance.

What are the complications of being underweight?

If you are underweight, you may be at greater risk of certain health conditions, including:

• malnutrition
• osteoporosis
• decreased muscle strength
• hypothermia
• lowered immunity

People can die from being very underweight.

Underweight females have less chance of becoming pregnant than those who are in a healthy weight range.

Resources and support

To find an accredited practising dietitian near you, or after-hours medical service, use the healthdirect Service Finder tool.

Visit Eat for Health for advice about healthy eating when you're older.

You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available to speak with you 24 hours a day, 7 days a week.

The Butterfly Foundation can also provide information and support for people with eating disorders. Call them on 1800 33 4673, 8am – midnight AEST, 7 days a week, chat online or email them.

If you or someone you care for has an eating disorder, talk to your doctor — it is important to seek treatment from a health professional.